The present invention relates to a method and apparatus for roasting coffee.
Three operations are needed to convert the green coffee beans into consumable beverage: (1) roasting, (2) grinding and (3) brewing. The characteristic flavour and aroma of coffee is developed only by roasting.
Roasting is a time-temperature dependent process, where chemical and a physical changes are caused in the green coffee beans accompanied by loss of dry matter primarily such as gaseous carbon dioxide and water and other volatile products of the pyrolysis. Roasting is normally carried out under atmospheric conditions with hot gases and excess air as primary heating agents. Heat also may be provided by contact with hot metal surfaces, usually as a supplement to convection from the hot gases. The degree of roast plays a major part in determining the flavour characteristic of extracts eventually brewed from roasted coffee, whatever the type.
In general, roasters have been designed and are available based on different mechanical principles as summarized below.
Horizontal rotating drum--solid or perforated wall PA0 Vertical static drum with blades PA0 Vertical rotating bowl PA0 Fluidized bed roaster PA0 Pressure roaster PA0 (a) charging a batch of nonroasted coffee beans to the roasting chamber; PA0 (b) subjecting the batch to the flow of air having the temperature required for bringing the beans of the batch to the pyrolysis; PA0 (c) establishing the point in time at which pyrolysis of said batch is started by at least one of the steps of: PA0 (d) when the commencement of the pyrolysis is established by the step (c), initiating the operation of a clocking device preset to a predetermined time length; PA0 (e) upon expiry of said predetermined time length generating an output signal for stopping the roasting operation and discharging roasted coffee from said roasting chamber. PA0 (a) a roasting chamber having a bottom portion and a top portion and operatively associated with heated air supply flowing in a direction from said bottom portion to said top portion; PA0 (b) a sound and/or temperature probe reaching into the roasting chamber; PA0 (c) said sound probe being adapted to sense the pattern of noise generated by the batch in said chamber as it is being roasted; PA0 (d) said temperature probe being adapted to sense the pattern of temperature increase of the surface of coffee beans in said roasting chamber; PA0 (e) said probe and/or probes being operatively connected to an timing device adapted to actuate a signal upon expiry of a predetermined time from a predetermined point of the pattern sensed by the respective probe, a clocking device adapted to actuate a roasting cycle ending device. PA0 (a) air circulation fan device disposed upstream of said roasting chamber and having a pressure side thereof connected to an upstream end of a hot air feeding duct, the downstream end of the hot air feeding duct being operatively connected with a hot air inlet of said roasting chamber at an upstream end of the chamber; PA0 (b) a hot air outlet system of said roasting chamber being disposed at a downstream end of the chamber and being connected to an upstream end of a return duct system, the downstream end of said return duct system being disposed at a suction side of said fan device; PA0 (c) said heated air circulation system being connected to an exhaust conduit system for discharging to the atmosphere at least a part of air present in said heated air circulation system; PA0 (d) said heated air circulation system being further connected to a downstream end of a fresh air supply conduit open at an upstream end thereof, a fresh air valve being disposed between the downstream and upstream ends of the fresh air supply conduit; PA0 (e) air heating device disposed within the heated air circulation system and adapted to maintain the temperature of the heated air in said chamber at a predetermined roasting temperature.
All roasters must also provide a cooling facility to bring the roasted coffee to ambient temperatures after the desired level of roast, usually by contact with cold air.
The known roasters use either "once through" flow of hot air or they recirculate the hot gases developed by roasting. The disadvantage of the first type is a high energy consumption since the incoming air has to be constantly heated to the required temperature. The second type is considered disadvantageous as it is difficult to properly control the roasting cycle. Another disadvantage of the circulated flow of air is in the danger of tar deposits formation on the roasted beans impairing the quality of the final product. The known roasters utilize the clocking of the roast time from the beginning of the process to its end. This is disadvantageous as when only one of many variables of the roasting process or conditions changes, the result is not uniform even for the same type. The changes in variables governing the roasting are inevitable. Every roast, even of the same type, is slightly different from another because of variables such as temperature or moisture.
In a coffee roasting operation, the original moisture present in green coffee beans is first removed. Then the roasting itself starts at a temperature of about 200.degree. C. after which through, exothermic reactions, escalation of the roasting process occurs which requires considerable control of the roasting for a given degree of roast. Reaction in green arabica coffee may start as low as 160.degree. C. The reaction peaks at about 210.degree. C. and falls off at about 250.degree. C.
The most obvious physical change to occur is the external color which ranges from light brown to almost black. This change is accompanied by exudation of oil to the surface with increased severity of roasts. Swelling of beans also progressively occurs.